1. Field of the Invention
This invention relates to a luminaire, in particular, a luminaire wherein switching power supply circuitry is used to operate an incandescent lamp at a dc voltage exceeding its rating.
2. Description of the Prior Art
Recently, the number of school children with visual fatigue or pseudomyopia and workers with visual fatigue is steadily increasing. This would be due to long-time TV and fine print watching under inappropriate illumination. Recent tendency prefers luminaires which make a person recognize the color of articles as they really are, and repeated discussions have been focused on color rendering properties of illumination.
Luminous sources which have been extensively used in luminaires are fluorescent and incandescent lamps. Fluorescent lamp has the merit that its relatively high total luminous flux provides a bright illumination, as well as having the demerits that its flicker tends to cause visual fatigue, and that the unbalance between total luminous flux and color temperature deteriorates color rendering properties to make articles look paler than they really are.
Although incandescent lamp is superior to fluorescent lamp because the well balance between total luminous flux and color temperature achieves satisfiable color rendering properties, incandescent lamp has the demerit that it is generally low in total luminous flux and this tends to cause visual fatigue when used in illumination for long time.
Japanese Patent Kokai No. 72,599/90 (Title: "Lighting device") and Japanese Patent Kokai No. 59,994/91 (Title: "Lighting device") disclose that in order to overcome these demerits of conventional luminaires, incandescent lamps are energized with a dc voltage exceeding their rating to obtain a light with a color temperature of about 2,900 K. or higher, desirably, in the range of about 2,950-3,100 K., which is flickerless, natural and gentle to the eyes.
The luminaires disclosed in these Japanese Patent Kokai however have the drawbacks that:
(i) they are not easily applicable in the illumination of facilities because the regulation of their power supply circuitry requires one individual power supply for every incandescent lamp; PA1 (ii) the size-down of their power supply circuitry is limited because they need relatively large-size smoothing capacitors; PA1 (iii) they are susceptible to voltage variation because the regulation of their output voltage is difficult; PA1 (iv) it is difficult to continuously or stepwisely change their output voltage for lamp dimming; and PA1 (v) to soft-start an incandescent lamp, one has to provide a relatively large-size overcurrent-limiting resistor in the current path including the incandescent lamp, and this resistor hinders the size-down of the luminaires, as well as generating heat. PA1 (i) the power supply can be easily made smaller because its power efficiency is high; PA1 (ii) two or more incandescent lamps can be easily operated with one power supply circuitry; PA1 (iii) the output voltage can be kept constant even when the input voltage and/or load current varies because the voltage regulation is easily achieved; PA1 (iv) the shortening in service life of incandescent lamp and damage of circuit elements can be effectively prevented because soft-start function can be easily imparted; PA1 (v) lamp dimming can be easily achieved by continuously or stepwisely changing the output voltage; and PA1 (vi) the power efficiency does not greatly vary even when the load power changes.